This invention relates to loudness control circuits in acoustic devices such as radio sets.
As is well known in the art, the hearing characteristic of a human being is such that, when the volume of sound is sufficiently large, then he can hear it substantially equally in quality or tone throughout the frequencies, but as the volume of sound is made smaller, it becomes difficult for him to hear the sound in the low and high frequency ranges.
Accordingly, the invention relates to the function of a loudness control circuit that, when the volume of sound is small, the low and high frequency ranges are automatically boosted to allow a person to hear the sound substantially uniformly in tone throughout the frequencies, and to a loudness control circuit in which the low and high frequency ranges are automatically most suitably boosted according to the variations in electric field strength of an FM broadcast wave.
A conventional loudness control circuit, as shown in FIG. 1, comprises: variable resistors 1 and 2; a CR circuit 3; a loudness switch 4; an input terminal 5; and an output terminal 6. The intermediate tap of the variable resistor 2 is connected to the CR circuit 3. When the slider of the variable resistor 1 is operated to reduce the volume, the high and low frequency ranges are boosted by the CR circuit 3 to make the volume and the quality of sound flat.
That is, in the conventional loudness control circuit, the volume and the quality of sound are adjusted by the variable resistors 1 and 2 and the CR circuit 3. Accordingly, when the loudness control circuit is inserted in an acoustic device, then the loss in gain is increased.
Furthermore, the loudness control circuit is disadvantageous in that, when the sliding contact of the variable resistor is set higher than the intermediate tap to increase the volume, the compensation effect of the loudness control circuit is inadequate.
In addition, it is impossible to provide the loudness control circuit employing the CR circuit in the form of an intergrated circuit.
Under the condition that, in receiving an FM broadcast signal, its electric field strength varies frequently, it is impossible for the conventional loudness control circuit in FIG. 1 to weight the loudness characteristic.
One example of the loudness control circuit will be described with reference to FIG. 2. When the volume is increased to the maximum value, then a flat frequency characteristic is obtained. Under this condition, the volume is gradually decreased. If, in this operation, the volume is boosted by 4 dB in the frequency ranges of 100 Hz and 10 KHz when the volume is decreased by 10 to 20 dB, and also the volume is boosted by 8 dB in the frequency ranges of 100 Hz and 10 KHz when the volumen is further decreased by 30 dB, then the person hears the produced sound flat in the quality of sound. The above-described data are typical one. That is, it goes without saying that the high and low frequency ranges can be boosted with the volume decreased as much as given levels.